1. Field of the Invention
This invention relates to a moving coil type stereophonic pickup cartridge of the type generally found in the U.S. Patent and Trademark Office under Class 369-136.
2. Description of the Prior Art
As described in U.S. Pat. No. 3,679,843, U.S. Pat. No. 3,299,219, and British Pat. No. 1,000,035, prior art stereophonic pickup cartridges of the moving coil type have been constructed such that a magnetic circuit is formed of a permanent magnet and pole pieces or yokes made of a high magnetic permeability material. At least one magnetic gap is provided in a part of the magnetic circuit, usually between the yokes. An armature on which moving coils are wound is attached to a rear portion of a cantilever. The cantilever is also provided with a stylus at the forward end portion and the armature is arranged in the magnetic gap. The stylus mounted on the cantilever traces the sound groove of a stereophonic disk record and, as the cantilever vibrates in accordance with the motions of the stylus, the coils of the armature move in the magnetic flux in the magnetic gap. Thus, electrical signals are induced in the moving coils corresponding to the undulations in the walls of the sound groove.
In order that the recorded sound in the high frequency range can be reproduced with high fidelity, the effective mass of the vibrational system i.e., the stylus, cantilever, armature and moving coils, is required to be reduced as much as possible. A direct consequence of reducing the mass of the coil is a generally smaller electrical output from the cartridge. The electrical output can be increased by increasing the density of the magnetic flux in the gap. This can be achieved by designing the magnetic gap to be narrower and/or increasing the magnetic power of the permanent magnet or other magnetic means employed to establish the magnetic flux in the gap. The increasing of the magnetic flux in the gap has usually resulted in an increase in the total mass of the cartridge which is attached to the tone arm. This increase in mass results in an increase in the moment of inertia of the tone arm at the stylus, causing poor tracking performance at the very low frequency range.
The very low frequency tracking problem is attributable to the general characteristics of disk records in as much as a disk record is typically made of a thermoplastic synthetic resin which may undergo warp when in storage or when handled improperly. A small but not ignorable number of the record disks commercially available have a center hole which is not concentric with the sound groove. As a result, unnecessary and harmful vibrations of the stylus tip may occur causing distortion of the reproduced sound. In the extreme case, the stylus disengages from the sound groove and skips across the disk usually damaging the disk and occasionally damaging the stylus. The harmful effects which are caused by the vibration of large amplitude and very low frequency ascribable to the warp and eccentricity of the disk are principally attributed to the fact that the moment of inertia measured at the stylus tip of the tone arm on which the cartridge is mounted is very large. Accordingly, the problems associated with large amplitude low frequency vibrations can largely be mitigated by reducing the mass of the tone arm and especially reducing the mass of the cartridge which contributes most to the moment of inertia at the stylus tip.
In order to reduce the mass of the cartridge as much as possible, it is necessary to reduce the mass of the permanent magnet or other source of magnetic flux and the associated yokes forming pole pieces. This has usually caused the electrical output generated through the moving coils to become unsatisfactorily small since the reduction of mass in the source of magnetic flux decreases the magnetic field intensity to an unsatisfactory level. As a result, it has been very difficult to improve the trackability in the very low frequency range of the cartridge through reducing the mass of the cartridge without also lowering the electrical signal output to a marginal level.
Thus, the principle object of the present invention is to provide a moving coil type stereophonic cartridge which has excellent trackability in the very low frequency range through reducing the effective mass of the tone arm but without lowering the electrical signal output. It is a further object of the present invention to keep the mass of the vibrational system as low as possible so that the recorded sound in the high frequency range can be reproduced with high fidelity.